1. Field of the Invention
The present invention relates to an addition-curable type silicone rubber composition, and more particularly to a silicone rubber composition with improved fluidity.
2. Description of the Prior Art
Addition-curable type silicone rubber compositions are in extensive use for such applications as electrically insulating potting material , PPC roll material, etc. These compositions generally comprise an alkenyl group-containing organopolysiloxane, an organohydrogen-polysiloxane containing an SiH group, a platinum catalyst and an inorganic filler. Specifically, for obtaining enhanced heat resistance, carbon black is used as an inorganic filler; for such purposes as improving thermal conductivity or preventing deformation of cured products due to thermal expansion, a silica or alumina having an average particle diameter of 1 .mu.m or above is used as an inorganic filler.
However, incorporation of carbon black into an addition-curable type silicone rubber composition markedly impairs the fluidity of the composition. In order to solve the problem, it has been proposed to use a carbon black of which particle surfaces have been treated with water and a hexaalkylsilazane (See Japanese Preexamination Patent Publication (KOKAI) No. 2-80461 (1990)). According to the proposed method, however, improvement of fluidity is not achievable, though the resulting silicone rubber compositions tend to show a lowered viscosity.
On the other hand, use of a silica or alumina having an average particle diameter of 1 .mu.m or above as an inorganic filler results in that the filler separates from silicone oil and sediments during storage of the silicone rubber composition. It is known that the separation and sedimentation can be prevented by addition to the composition of fumed silica having a specific surface area of 50 m.sup.2 /g or above. The addition of such fumed silica, however, involves the problem that fluidity of the composition is lowered. As a means for overcoming the above drawbacks and obtaining an addition-curable silicone rubber composition improved in thermal conductivity and other properties, Japanese Patent Publication (KOKOKU) No. 52-22965 (1977), for example, discloses a process which comprises using a filler such as alumina, magnesium oxide or the like and treating the filler-loaded curable composition with water and a silazane compound. However, such a treatment requires a heating step, and, in addition, another heating step is needed to remove excess portions of the water and silazane compound from the reaction system. Thus the process is disadvantageous from the viewpoint of productivity and cost.